1. Field of the Invention
The present invention relates to blocked polyisocyanates containing at least one reversibly blocked isocyanate group and at least one isocyanate group in the form of a urea group, which may be converted to a thermally stable hydantoin group at elevated temperatures, and to their use, optionally in combination with compounds containing isocyanate-reactive groups, especially in electrodeposition coating applications.
2. Description of the Prior Art
Coating compositions containing blocked polyisocyanates are commonly used in the coating industry for the production of one-component coating compositions. Because the isocyanate groups are blocked, they are not reactive under ambient conditions with the isocyanate-reactive component present in the coating composition. However, when the composition is heated to elevated temperatures the blocking agent is released to reform isocyanate groups, which then react with the isocyanate-reactive component to form a coating.
In certain blocked polyisocyanates used in the coatings industry both of the isocyanate groups of the starting polyisocyanate are not reacted with blocking agents. For example, in blocked NCO prepolymers the diisocyanate in the terminal position has one blocked isocyanate group and one isocyanate group which is reacted with, e.g., a polyol. Alternatively, in the production of electrodeposition coatings one of the isocyanate groups of 2,4-diisocyanatotoluene is blocked with a blocking agent, such as a primary alcohol, while the remaining isocyanate group is either reacted with a high molecular weight polymer having pendant amino and/or hydroxyl groups or it is reacted with a low molecular weight compounds having such groups and blended with the high molecular weight polymer.
One of the disadvantages of these compositions is that during the final cure at elevated temperatures the isocyanate group attached to the polymer unblocks (i.e., converts to an isocyanate group and a hydroxyl or amino group) at approximately the same temperature as the blocked isocyanate group becomes unblocked. When both sides become detached (unblocked), the diisocyanate monomer, e.g, TDI, tends to migrate to the surface, where it can cause discoloration, e.g, of subsequently applied coating layers.
It is object of the present invention to overcome this problem by providing blocked polyisocyanates in which it is possible to unblock the blocked isocyanate group without unblocking any isocyanate groups which are not intended to be unblocked.
This object may be achieved with the blocked polyisocyanates according to the present invention described hereinafter.